This invention relates generally to the field of hydrodynamic bearings, and more specifically to a design comprising two spindle bearings, one gap/air and one fluid in order to provide balance and reduced power consumption.
Disc drives are capable of storing large amounts of digital data in a relatively small area. The disc drives store information on one or more spinning recording media. The recording media conventionally takes the form of a circular storage disk in a plurality of concentric circular recording tracks. A typical disk drive has one or more disks for storing information. This information is written to and read from the disks using read/write heads mounted on actuator arms that are moved from track to track across surface of the disk by an actuator mechanism.
Generally, the disks are mounted on a spindle that is turned by a spindle motor to pass the surfaces of the disks under the read/write heads. The spindle motor generally includes a shaft supporting from a base plate and a hub to which the spindle is attached having a sleeve into which the shaft is inserted. Permanent magnets, which are typically attached to the hub, interact with a stator winding to rotate the hub relative to the shaft. This description is consistent with a fixed shaft motor; however, the invention to be described below is as easily useable with a motor comprising a rotating shaft, an end of the shaft supporting the hub for rotation to support the rotation of the disks.
In either case, to facilitate rotation, one or more bearings are disposed between the hub or sleeve and the shaft.
Over time, disk drive storage density has tended to increase, and the size of the storage system has tended to decrease. This trend has led to greater emphasis on restrictive tolerances in the manufacturing and operation of magnetic storage disk drives. For example, to achieve increased storage density, read/write heads must be placed increasingly close to the surface of the storage disk.
As a result, the bearing assembly which supports the storage disk is of critical importance. A typical bearing assembly of the prior art comprises ball bearings supported between a pair of bearing paces which allow a hub of a storage disk to rotate relative to a fixed member. However, ball bearing assemblies have many mechanical problems such as wear, run-out and manufacturing difficulties. Moreover, resistance to operating shock and vibration is poor because of damping.
An alternative bearing design is a fluid dynamic bearing. In a fluid dynamic bearing, lubricating fluid such as air or liquid provides a bearing surface between a fixed member of the housing (e.g., the shaft) and a rotating member which supports the disk hub. Typical lubricants include oil or similar hydrodynamic fluids. Hydrodynamic bearings spread the bearing interface over a large surface area in comparison with a ball bearing assembly, which comprises a series of point interfaces. This is desirable because the increased bearing surface reduces wobble and run-out between the rotating and fixed members. Further, the use of fluid in the interface area imparts damping effects to the bearing which helps to reduce non-repeat run-out.
It is also known that the stiffness to power ratio is a primary way of measuring the efficiency of the spindle bearing assembly. Most known fluid dynamic bearings today in commercial use are made with oil as the fluid which is maintained in the gap between the two relatively rotating surfaces. This maintains the stiffness of the bearing, that is the resistance to shock and vibration; however, because of the relatively high viscosity of such fluids, which at lower temperatures, such as at startup, considerable power is consumed to establish and maintain high speed rotation.
It is an objective of the present invention to provide a bearing system in which the stiffness is maintained while the power consumption necessary to establish and maintain rotation of the bearing system is reduced.
It is a further objective of the invention to provide a bearing system in which a relatively stiff, low power system is achieved without utilizing fluid dynamic bearings with extremely small gaps in all embodiments.
These and other objectives of the invention are provided in a bearing system in which an axially stiff narrow gap fluid dynamic gas bearing is preloaded by an axially less stiff larger gap fluid dynamic bearing. More specifically, pursuant to the present invention a system is provided in which two fluid dynamic bearings are provided spaced apart along a shaft, one of the bearings comprising a fluid dynamic bearing, the other comprising an air bearing. In this exemplary embodiment, the fluid dynamic bearing has a larger gap, while the air bearing has a relatively small gap so that power consumption is diminished while stiffness is maintained.
According to embodiments of the present invention, the air bearing is substantially larger in size than the fluid bearing; the gap of the air bearing may be as much as one-fourth of the gap in the fluid bearing. The overall surface area of the air bearing may be twice as much or more than the working surface area of the fluid bearing.
In a typical embodiment, each of the bearings is a conical bearing comprising a cone supported on the shaft, with a surrounding sleeve providing a surface facing an outer surface of the cone, the gaps being defined between the surfaces of the cone and the sleeve. An alternative embodiment, one of the fluid dynamic bearings may be conical bearing, and the other a thrust plate bearing design.
Other features and advantages of the invention and alternative approaches will be apparent to a person of skill in the art who studies the following exemplary embodiments of the invention, given with reference to the following drawings.